Interpretive Summary: The development of domestic natural rubber producing crops in the United States is progressing with traditionally-bred lines of plants (primarily guayule) with high rubber yield. Truly sustainable industrial agriculture will require more significant improvements in the amount of rubber produced by biosynthesis. Molecular approaches offer the most promise for making substantial improvements in rubber yield and quality in alternative rubber-producing plant species. A combination of biochemistry-based approaches, and genomics and proteomics methods capitalizing on model systems, promise to generate the fundamental breakthroughs in functional understanding needed to fully exploit molecular methods and generate significantly improved lines. In addition, improvements in tissue culture and transformation methods, especially designed for recalcitrant species, are being aggressively pursued by academic, federal, and industrial laboratories, and their success should greatly facilitate the development and introduction of additional rubber-producing crops.

Technical Abstract:
Natural rubber (cis-1,4-polyisoprene) is an essential plant derived raw material required for the manufacture of numerous industrial and medical related products. This elastic polymer is synthesized and sequestered within cytosolic vesicles known as rubber particles. When provided with farnesyl-pyrophosphate (FPP) and isopentenyl-pyrophosphate, rubber particles alone are sufficient for rubber biosynthesis, indicating that these vesicles must possess the required protein machinery for rubber production. As classic biochemical approaches have failed to identify rubber biosynthetic proteins or genes, we have taken an alternative proteomic- and genomic-based approach with four rubber producing plant species including Hevea brasiliensis, Parthenium argentatum, Taraxacum kok-saghyz, and Ficus elastica. From these studies, we have identified a common set of rubber particle-associated proteins and a subset of candidate rubber biosynthetic proteins. Through reverse genetic analyses in Taraxacum kok-saghyz, we have functionally identified a number of candidate genes whose altered expression appears to affect rubber yields and/or polymer length. We will discuss the potential of these genes for the improvement of current rubber crops and the development of domestic rubber crops species.